1. Field of the Invention
The present invention generally relates to a method for producing a permanent magnet that is applicable for use in motors and actuators of various types, and more particularly, the present invention relates to an iron-based rare earth magnet including multiple ferromagnetic phases and a method for producing such a novel magnet.
2. Description of the Related Art
Recently, it has become more and more necessary to further improve the performance of, and further reduce the size and weight of, consumer electronic appliances, office automation appliances and various other types of electric equipment. For these purposes, a permanent magnet for use in each of these appliances is required to maximize its performance to weight ratio when operated as a magnetic circuit. For example, a permanent magnet with a remanence Br of 0.5 T or more is now in high demand. Hard ferrite magnets have been used widely because magnets of this type are relatively inexpensive. However, the hard ferrite magnets cannot achieve the high remanence Br of 0.5 T or more.
An Sm—Co type magnet, produced by a powder metallurgical process, is currently known as a typical permanent magnet that achieves the high remanence Br of 0.5 T or more. However, the Sm—Co type magnet is expensive, because Sm and Co are both expensive materials
As for the Nd—Fe—B type magnet on the other hand, the magnet is mainly composed of relatively inexpensive Fe (typically at about 60 wt % to 70 wt % of the total weight), and is much less expensive than the Sm—Co type magnet Examples of other high-remanence magnets include an Nd—Fe—B type sintered magnet produced by a powder metallurgical process and an Nd—Fe—B type rapidly solidified magnet produced by a melt quenching process. An Nd—Fe—B type sintered magnet is disclosed in Japanese Laid-Open Publication No. 59-46008, for example, and an Nd—Fe—B type rapidly solidified magnet is disclosed in Japanese Laid-Open Publication No. 60-9852, for instance. Nevertheless, it is still expensive to produce the Nd—Fe—B type magnet. This is partly because huge equipment and a great number of manufacturing and processing steps are required to separate and purify, or to obtain by reduction reaction, Nd, which usually accounts for 10 at % to 15 at % of the magnet. Also, a powder metallurgical process normally requires a relatively large number of manufacturing and processing steps by its nature.
Compared to an Nd—Fe—B type sintered magnet formed by a powder metallurgical process, an Nd—Fe—B type rapidly solidified magnet can be produced at a lower cost by a melt quenching process. This is because an Nd—Fe—B type rapidly solidified magnet can be produced through relatively simple process steps of melting, melt quenching and heat treating. However, to obtain a permanent magnet in bulk by a melt quenching process, a bonded magnet should be formed by compounding a magnet powder, made from a rapidly solidified alloy, with a resin binder. Accordingly, the magnet powder normally accounts for at most about 80 volume % of the molded bonded magnet. Also, a rapidly solidified alloy, formed by a melt quenching process, is magnetically isotropic
For these reasons, an Nd—Fe—B type rapidly solidified magnet produced by a melt quenching process has a remanence Br lower than that of a magnetically anisotropic Nd—Fe—B type sintered magnet produced by a powder metallurgical process
As disclosed in Japanese Laid-Open Publication No. 1-7502, a technique of adding, in combination, at least one element selected from the group consisting of Zr, Nb, Mo, Hf, Ta and W and at least one more element selected from the group consisting of Ti, V and Cr to the material alloy effectively improves the magnetic properties of an Nd—Fe—B type rapidly solidified magnet. When these elements are added to the material alloy, the magnet has increased coercivity HcJ and anticorrosiveness. However, the only known effective method of improving the remanence Br is increasing the density of the bonded magnet. Also, where an Nd—Fe—B type rapidly solidified magnet includes a rare earth alloy at 6 at % or more, a melt spinning process, in which a melt of its material alloy is ejected against a chill roller, has often been used in the prior art to rapidly cool and solidify the material alloy at an increased rate.
As for an Nd—Fe—B type rapidly solidified magnet, an alternative magnet material was proposed by R. Coehoorn et al., in J. de Phys, C8, 1998, pp. 669-670. The Coehoorn material has a composition including a rare earth element at a relatively low mole fraction (i.e., around Nd38Fe772B19, where the subscripts are indicated in atomic percentages); and an Fe3B phase as its main phase. This permanent magnet material is obtained by heating and crystallizing an amorphous alloy that has been prepared by a melt quenching process. Also, the crystallized material has a metastable structure in which soft magnetic Fe3B and hard magnetic Nd2Fe14B phases coexist and in which crystal grains of very small sizes (i.e., on the order of several nanometers) are distributed finely and uniformly as a composite of these two crystalline phases For that reason, a magnet made from such a material is called a “nanocomposite magnet” It was reported that such a nanocomposite magnet has a remanence Br as high as 1 T or more. But the coercivity HcJ thereof is relatively low, i e., in the range from 160 kA/m to 240 kA/m. Accordingly, this permanent magnet material is applicable only when the operating point of the magnet is 1 or more
It has been proposed that various metal elements be added to the material alloy of a nanocomposite magnet to improve the magnetic properties thereof. See, for example, Japanese Laid-Open Publication No. 3-261104, Japanese Patent Publication No. 2727505, Japanese Patent Publication No. 2727506, PCT International Publication No. WO 003/03403 and W. C. Chan et. al., “The Effects of Refractory Metals on the Magnetic Properties of α —Fe/R2Fe14B-type Nanocomposites”, IEEE Trans. Magn. No. 5, INTERMAG. 99, Kyongiu, Korea, pp. 3265-3267, 1999. However, none of these proposed techniques are reliable enough to always obtain a sufficient “characteristic value per cost”. More specifically, none of the nanocomposite magnets produced by these techniques realizes a coercivity high enough to actually use it in various applications. Thus, none of these magnets can exhibit commercially viable magnetic properties.